Late on 2012 July 23, the STEREO-A spacecraft encountered a fast forward shock driven by a coronal mass ejection (CME) launched from the Sun earlier that same day. The estimated travel time of the disturbance (~20 hr), together with the massive magnetic field strengths measured within the ejecta (100 nT), made it one of the most extreme events observed during the space era. In this study, we examine the properties of the shock wave. Because of an instrument malfunction, plasma measurements during the interval surrounding the CME were limited, and our approach has been modified to capitalize on the available measurements and suitable proxies, where possible. We were able to infer the following properties. First, the shock normal was pointing predominantly in the radial direction (). Second, the angle between and the upstream magnetic field, θBn, was estimated to be ≈34°, making the shock "quasi-parallel," and supporting the idea of an earlier "preconditioning" ICME. Third, the shock speed was estimated to be ≈3300 km s?1. Fourth, the sonic Mach number, Ms, for this shock was ~28. We support these results with an idealized numerical simulation of the ICME. Finally, we estimated the change in ram pressure upstream of the shock to be ~5 times larger than the pressure from the energetic particles, suggesting that this cosmic-ray modified shock had not reached steady-state, but instead, had been caught in an early, transient phase in its evolution.
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